In a large saucepan or small stockpot over medium heat, heat the oil. Add the celery and onion and cook, stirring occasionally, for 4 minutes.
Add the fennel seeds, carrots, and parsnips and continue to cook, stirring occasionally, until the onion is tender, about 4 minutes.
Add the stock, bring to a boil over high heat, lower the heat to a simmer, and cook until all the vegetables are tender, about 5 minutes.
Add salt and pepper to taste. Serve hot, garnished with the scallions.
